1. Field of the Invention
The present invention concerns a method for excitation of a magnetization in the generation of MR angiography images with the TOF (time-of-flight) technique, and an MR system for implementing such a method.
2. Description of the Prior Art
Time-of-flight magnetic resonance angiography (TOF MR angiography) is a non-invasive imaging method for depiction of the vessel structure of an examination subject. It is based on the inflow of “fresh” spins, which have not been pre-saturated, into an imaging plane or, respectively, into an imaging volume. The stationary magnetization, i.e. the stationary spins of the imaging plane, is saturated by the repeated excitation in a short time interval TR. The signal of this magnetization is largely suppressed while the magnetization that has not been pre-saturated (which results via the blood flow into the imaging plan during the acquisition, for example) exhibits a high signal proportion.
The advantage of TOF MR angiography compared to contrast agent-intensified MR angiography is that it is not invasive, i.e. it does not use a contrast agent. Although contrast agent-intensified angiography methods have a wide range of use, contrast agents are not permitted for angiography in all countries. Furthermore, contrast agent-intensified angiography imaging methods should not be used for examination subjects with renal insufficiency.
TOF MR angiography has the disadvantage that the contrast between vessels and stationary tissue depends on the penetration depth of the unsaturated magnetization in the imaging plane. This means that the contrast is dependent on the blood flow speed and the orientation of the blood vessels to be depicted relative to the imaging plane. The best contrast is achieved when the vessel runs perpendicular to the imaging plane. As in stationary tissues, no unsaturated spins with high signal proportion can be resupplied into vessels that run parallel to the imaging plane, such that these vessels lose their contrast relative to the static environment within a certain vessel length. One possibility to minimize this contrast loss is to sequentially acquire only very thin individual slices so that the course of a vessel within an imaging plane is minimized. However, this individual slice method leads to a poor signal-to-noise ratio that can be achieved in a predetermined acquisition time. Furthermore, the achievable resolution perpendicular to the imaging plane is limited given the acquisition of individual slices.
In 3D acquisitions with volume excitation and two phase coding gradients, it is known to reduce the decreasing contrast in the flow direction in that a flip angle gradient perpendicular to the imaging plane is used, which leads to the situation that spins that have flowed further into the volume experience an increasing flip angle, and therefore contribute to more signal in order to reduce the decreasing magnetization in the vessels due to saturation effects (see D. Atkinson et al. in “Improved MR Angiography: Magnetization Transfer Suppression with Variable Flip Angle Excitation and Increased Resolution” in Radiology, 1994: Vol. 190, Pages 890-894).
The signal in the vessels remains thus can be made essentially constant across the volume, when the blood flow direction is perpendicular to the imaging plane. However, when the vessel does not run perpendicular to the imaging plane, the contrast of the vessels is reduced by saturation effects relative to the stationary environment, as mentioned above.